Higher beings aren't immortal; they only have an extremely long life span. They can still be killed, hurt, and reincarnated like other beings.
There are only three known beings that are said to be immortal are...
one—the creator of the world,
two—the world tree,
and lastly, the demon king.
Loki is a demonic god, but he was not throned as the demon king.
Loki existed in this world longer than anyone else, but he was not the one who created it and he is not greater than the supreme god.
So which part of it is Loki?
What makes him immortal?
No one knows how he became immortal, and his purpose in existence cannot be defined.
Watson tried to defeat Loki centuries ago, but was unable to do so after discovering that no matter how much he tried to defeat the demonic god, Loki never dies. Thus, you only receive misfortune if you catch the god of mischief's attention.
